1. Field of Invention
This invention relates to ocean current turbines and, more specifically, to a floating tower frame turbine system to generate electrical power and/or to produce fresh water from seawater.
2. Description of Related Art
Ocean currents are a major, largely untapped energy resource. Research and development in this area is driven by the need to generate electricity from renewable energy resources, particularly in view of the rising level of CO2 in the earth's atmosphere from combustion of carbon fuels and the resulting impact on climate from global warming.
An ocean current is a continuous, directed movement of seawater generated by the forces acting upon this flow, such as breaking waves, wind, Coriolis effect, evaporation, temperature and salinity differences (cabbeling), and tides caused by the gravitational pull of the Moon and the Sun. Depth contours, shoreline configurations and interaction with other currents influence a current's direction and strength. Ocean gyre currents are relatively constant and generally flow in one direction, in contrast to periodic tidal currents that reverse in flow direction due to gravitational forces. While ocean currents move slowly relative to typical wind speeds, they carry a great deal of energy because of the density of water, which is more than 800 times that of air. The following table illustrates the average power density as a function of wind or current flow speeds for a wind turbine or marine turbine, respectively.
Wind TurbineMarine TurbineAverage WindAverage FlowSpeed (typicalAverage PowerSpeed (typicalAverage Powerrange-m/s)Density W/M2range - m/s)Density W/M26132.994987.52581.208868.53761.431500106131.602101
With gyre currents, the constancy of flow also provides the opportunity for added energy capture, compared to the intermittency of wind. Because of these physical properties, ocean currents contain an enormous amount of energy that can be captured and converted to a usable form.
The United States, the United Kingdom, Japan, and other countries are pursuing ocean current energy; however, marine current energy is at an early stage of development. Relative to wind, the energy potential wave and tidal resources is the least understood and its technology is the least mature. Commercial grid-connected ocean turbines are in the infancy and only a small number of prototypes and demonstration units have been tested.
For ocean current energy to be utilized successfully at a commercial scale, a number of engineering and technical challenges need to be addressed, including: avoidance of blade cavitation (bubble formation); prevention of marine growth buildup; reliability (since at-sea maintenance costs are potentially high); efficient methods of deployment; corrosion resistance; and anchoring and mooring methods. System reliability is of particular importance, since the logistics of at-sea maintenance is likely to be limited by accessibility, and windows of acceptable weather and sea-states conditions, therefore the costs can be high. Furthermore any system deployed in the ocean must be able to survive large waves and storms, raising the capital cost and maintenance. Moreover, an ocean current turbine system must have minimal impact on the marine environment, such as fishing grounds and beach shoreline, and be with ocean navigation.
Korean Patent No. 936907 to Kim discloses an ocean floor mounted, two rotor tidal generator system in which a main body automatically rotates so that a rotor always faces the tidal flow. This system is expensive due to the structural requirements necessary for dealing with the overturning moment of the whole structure from thrust load of the current on the rotors. This limits deployment to shallow locations. Installation is costly since it is only possible during short periods between tidal flows. This permanent installation precludes returning the structure to the shore base facility for long-term servicing. Servicing just one rotor results in raising all the rotors above the surface and shutting down all of the rotors (not just the one requiring servicing) of the entire system resulting in significant lost production. Moreover, the design requires both rotors to operate simultaneously since a shutdown of one rotor would yaw the rotor support structure toward alignment with the flow rather than squarely facing the flow, significantly reducing production.
U.S. Pat. No. 7,307,356 to Fraenkel discloses a dual rotor marine current turbine mounted on the ocean floor. This system is also expensive due to structural requirements to deal with the overturning moment of the whole structure from thrust load of the current on the rotors. Installation and securing to the ocean floor is only possible during short periods between tidal flows. Rotors and support structure can be raised for servicing, however all power generation is shut down if only one rotor requires servicing. Rotors and support structure do not yaw for change in tidal flow direction.
Great Britain Patent No. 2,447,774 to Fraenkel discloses a deep water current turbine system anchored to an ocean floor. If one rotor malfunctions, requiring servicing, the entire system must be shut down and brought to the surface. In a tidal flow, this would be difficult, as the flow in one direction drops off and waters calm, providing a brief window for servicing operations before the flow reverses, at which point the whole floating structure swings around to an opposite position on the surface due to the opposite flow direction. This design appears costly, complex, and dangerous. It also requires a much larger operating footprint represented by the full surface of the operating water column perimeter, which results in higher power collection cabling costs, lower power generating density per unit of seafloor area, and the potential for rotor entanglement with the mooring lines.
These prior art systems are not capable of producing cost-effective, utility-scale electrical power output to meet modern energy needs. What is needed is a system for efficiently capturing power from ocean or tidal currents, to generate electric power or produce desalinated water, which is cost effective to manufacture, deploy, and maintain.